Coexistence of multiple charge-density waves and superconductivity in SrPt2As2 revealed by 75As-NMR/NQR and 195Pt-NMR

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The relationship between charge density wave (CDW) orders and superconductivity in arsenide superconductor SrPt₂As₂ with Tc = 5.2 K which crystallizes in the CaBe₂Ge₂-type structure was studied by ⁷⁵As nuclear magnetic resonance (NMR) measurements up to 520 K, and ⁷⁵As nuclear quadrupole resonance (NQR) and ¹⁹⁵Pt-NMR measurements down to 1.5 K. At high temperature, ⁷⁵As-NMR spectrum and nuclear spin relaxation rate (1/T₁) have revealed two distinct CDW orders, one realized in the As-Pt-As layer below T CDW As(1) = 410 K and the other in the Pt-As-Pt layer below T CDW As(2) = 255 K. The 1/T₁ measured by ⁷⁵As-NQR shows a clear Hebel-Slichter peak just below Tc and decreases exponentially well below Tc. Concomitantly, ¹⁹⁵Pt Knight shift decreases below Tc. Our results indicate that superconductivity in SrPt₂As₂ is in the spin-singlet state with an s-wave gap and is robust under the two distinct CDW orders in different layers.